Transporting liquefied gas in combination with crude oil



April 19, 1966 R. may 3,

TRANSPORTING LIQUEFIED GAS IN COMBINATION WITH CRUDE OIL Filed March 5, 1964 I 2 Sheets-Sheet 1 INVENTOR RICHARD RIGBY HIS ATTORNEY A ril 19, 1966 R. RIGBY 3,246,480

TRANSPORTING LIQUEFIED GAS IN COMBINATION WITH CRUDE OIL Filed March 5, 1964 2 Sheets-Sheet 2 INVENTOR RICHARD RIGBY HIS ATTORNEY United States Patent 3,246,480 TRANSPQRTING LIQUEFIEDv GAS IN COMBINATION WITH- CRUDE OIL Richard. Rigby, Ledsham, Wirral, England, assignor to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed Mar. 3, 196 4, Ser. No. 348,975 Claims priority, application-G/igeat- Britain, Mar. 5, 1963,

. 3 5 Claims. (Cl. 62;48)

This invention relates to the transport and storage of liquefied gases, that is to say liquids at or near their boiling .point, andparticularly but not exclusively, to the transport and storage of liquefied petroleum gases, such as liquid butane.

Although it is known to, avoid to some extent the disadvantages of'evaporati've loss of volatile liquids such as gasoline during transport and storage by dissolving these liquids in a heavy product such as crude petroleum, difficualty is encountered when it is attempted to extend this idea to liquefied gases. and heavy products. In many cases the difference between, the temperature of the liquefied gas and the heavy product is, at least initially, so, great that mixing of the two. components. results in substantial evaporation of the, liquefied gas.

It has now been found that the above-mentioned difficul-ty can be substantially avoided by means of, the present invention. Accord-ing to the invention, a. liquefied gas is transported or stored in association. with a liquid having a substantially lower vapor pressure in an amount such that if the two liquids were completely mixed with each other the vapor pressure of the mixture, would ex- Ceed atmospheric pressure at transport, or storage temperatures- By floating or supporting the liquefied gas on, al body of liquidv having a substantially lower vapor pressure heat, transfer from the carrier liquid to the liquefied gas is minimized without recourse to. specially insulated vessels or to the application of pressure.

Broadly, the present invention relates to a method of transporting or storing liquefied gas which comprises initially floating or supporting said liquefied gas. on a carrier liquid having a temperature not exceeding 60 C.; a. vapor pressure not exceeding on atmosphere at the prevailing temperature; a density greater than that of said liquefied gas. at its boiling point. and which is chemically inert towards said liquefied gas; condensing any of said liquefied gas which. vaporizes; and, returning the condensate to the body of said floating or supported liquefied gas. in. some cases th liquefied gas will remain in transport or storage for a time and; at a temperature at which it will dissolve entirely in the carrier liquid.

In other words, the present invention provides a method of transporting or storing liquefied gases by placing them in thermal contact with a heavy product or carrier liquid at a time when the temperature of the carrier liquid is so much higher than the boiling point of theliqucfied gas that mixing. would be. ineifective. By forming a relativelly compact body of liquefied gas, the transfer of heat from; the carrier liquidis reduced, as compared with that during complete mixing, to the extent that the evaporativ loss of the liquefied. gas is kept within economic bounds. In this way the carrier liquid acts as an insulating support or container for the body of liquefied gas, thus reducing or eliminating the necessity for fixed insulated containers. Moreover, according to one aspect of the invention, by reason of contact between the liquefied gas and the carrier liquid, any fall in temperature towards the boiling point of the liquefied gas will result in increasing amounts thereof dissolving in the .carrier liquid, thus further reducing the evaporative loss of the liquefied gas.

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Accordingly, the present invention also provides. a method for the, transport or storage of liquefied, gas over a period during which, the temperature changes, which method. comprises maintaining the, liquefied gas in. con tact, with a carrier liquid as hereinbefore. defined and. having the liquefied g s p esent as a relatively compact, body of liquid floating on. or supported by the carrier liquid when. the. temperature of this liquid is, substantially higher thanthe boiling point of theliquefiedgas, and having the. liquefied, gas dissolved in the. carrier liquid at lower temperatures. I

'Examples. of, liquefied. gases, which. may be transported or stored in accordance with the present invention are acetylene, ethylene, ethane, propane, .pr-opene, n-butane, iso-butane, n -butene, iso,.-.butene1 and mixtures. thereof. The method of, the present invention is particularly suitable for the transport and storage, of liquefied petroleum gases comprising three and four carbon atoms per molecule and of mixtures of such gases. Examples of carrier liquids which may. be, used. in accordance. with the present invention are crude petroleum, topped crude, residual fuel oil, lubricating oil. dist ipllates, catalystically cracked cycle oil, thermally.- cracked gas oil, straight run gas oils, kerosenes, heavy gasoline fractions and mixtures thereof. Preferred carrier liquids are crude. petroleum, topped crude and; residual fuel oil.

The present invention may be. carried: into eifect merelyby supplying liquefied gas in any, convenient manner to the surface of a carrier liquid contained in a cargo or storage. tank; equipped withmeansfor condensing boil off vapors. The condensing means may compriseany known type; of condenser capable of opera-ting continuously at temperatures suitably below the boiling point of the liquefied gas, It is aparticularly advantageous aspect of the present invention, however, that said; con,- denser system may only be working at its maximum capac ilt for .aportion Of; the. time during which the l'iqufiedgas is; being transported or stored. Thus, the presence of thefioating layer or body ofi liquefied gas on the surface of the carrier liquid will cool the carrier liquid. This cooling. may be; assisted or augmented; by a fall in; ambient temperature, such, for example as; will occur it acargo comprising liquefied gas and carrier liquid is loaded; into a tanker; in an. area of high ambient temperature and. unloaded in a cooler area.v As the tempe r r tthe arrier liqu f lls. it vap p u e al =fallsand it is consequently capable of dissolving more liquefied gas; The; datav of Table I relate, to a crude petroleum having avapor'pressure of 1 atmosphere at a temperature of; 1 00, F. and; are: given; byway of example to indicate how the solubility of liquefied; butane in, crude petroleum increases as the temperature of the; crude petroleum decreases.

The data of Table II are similarly given way oi'examp e. and relate, to. the solubility of liquefied propane. in

crude petroleum.

Table II Temperature of crude petroleum, :20. 1'

Solubility of propane, percent; volume on a q i iquid vOlume basis 0 1 2 Thus it can arise that a crude petroleum at a tem- (not shown) of refrigerant.

perature of 100 F. whch is incapable of dissolving nbutane without raising the vapor pressure of the mixture to a value above atmospheric pressure, may dissolve up to nearly 20% by volume of n-butane when its temperature has fallen to 60 F., the vapor pressure of the resulting mixture being not greater than atmospheric pressure. Such a cargo. may be discharged at the unloading port in themanner normally applicable to crude petroleum and I it is accordingly advantageous so to choose the amount of liquefied gas to be transported that it will all have dissolved in. the carrier liquid before the cargo comprising liquefied gas and carrierliquid has arrived at its destination. If, howeven'the liquefied gas does not all dissolvein the carrier liquid, then special pumping arrangements will be. required for discharging the liquefied gas still floatingon thesurface of the carrier liquid. Such arrangements may comprise, for'example, a submerged pump provided wi-th a floating suction, or a gas lift system provided with a float-ing'suction.

" A particularly advantageous way of carrying the prescut-invention into effect is to retain the floating liquefied gas within an open topped retainer, which has the effect of maintaining the body of liquefied gas in com pact form and also prevents contact of the liquefied gas Withthe walls of the vessel containing the carrier liquid thereby avoiding chilling and embrittlernent of the metal forming this container. Alternatively, the retainer may take the form of a pan constrained to float on the surface of the" carrier liquid between guides which permit vertical but not lateral displacement of the pan, in which case discharge of the liquefied gas therein contained will generally necessitate provision of pumping arrangements separate from those concerned'with discharge of the carrier liquid. It is to be understood that dissolution of liquefied gas in the carrier liquid may take place by virtue of vapor contact of the liquefied gas with the carrier liquid. I i

' If desired, a cargo comprising aliquefied gas and a carrier liquid may be stored after unloading in the manner provided by the present invention,for example in a" tank provided with a retainer and means for condensing boil offvapor as hereinhefore described.

' The invention will now'be described with reference to "the accompanying drawings wherein:

' FIGURE 1 is a diagrammatic view of a section through a marine tanker taken in a plane perpendicular to the length of theship'to show a center 'cargo't'ank provided with acondenser and containing liquefied gas and a -form of 'a skirt and containing liquefied gas and a carrier liquid in accordance with the present invention;

FIGURE 3 is a diagrammatic form of a section (taken in 'the same plane as FIGURE 1) through a center cargo tank provided with a condenser and a retainer in the form of a floating pan constrained by guides and contaning liquefied gas and carrier liquid-in accordance with the present invention; and

FIGURE 4 is a diagrammatic view of a section (taken in the same plane as FIGURE 1) through a center cargo Jt-ank provided with a condenser and retainer in accordance with the present invention and containing liquefied gas and a carrier liquid and providedwith means for discharging liquefied gas from the surface of the carrier liquid.

Referring now to FIGURE 1, a center cargo tank 1 is flanked by wing cargo tanks 2 and 3, the three tanks being closed by means of a roof 4 which also comprises the deck of the vessel. The tank 1 is provided with a vent 5 which gives access to a condenser 6. A cooling coil 16 of the condenser 6 'has an entry conduit 7 and an exit conduit 8 connected to a central source A condenser vent 9 is -carrier'liq'uidin accordance with the present invention;

connected to a central conduit (not shown) which also receives the vents from othep condensers of the tankage system and finally gives access to a mast-head vent. With the condenser 6 in operation the tanks 1, 2 and 3 are supplied with crude petroleum 10 through the normal loading ports (not shown) of the tanks. Allowance is made in the center tank 1 for the volume of liquefied gas to be carried. Liquefied butane 12 is then admitted slowly to the'surface of the carrier liquid 10 in center tank 1 by means of a swing-arm the swingarm being slowly raised during delivery to insure that the liquefied butane 12 is delivered to the surface of the carrier liquid 10 with aminirrium of kinetic energy. During the. early part of the delivery the rate of boilofi of the liquefied butane 12 is high and the condensate is returned to the surface of thecrude petroleum 10 by the vent 5. There the condensate, together with the remained of the incoming liquefied butane, gradually forms a floating layer 12 on the surface of the crude petroleum 10. The floating layer 12 of liquefied butane cools thesurface of the crude petroleum 10 and as the temperature of the bulk of the crude petroleum in the center tank 1 falls, due to cooling by the liquefied butane and to any decrease in ambient temperature, the vapor pressure of the crude petroleum falls and dissolution of the liquefied butane in the crude petroleum takes place. As a result of the accompanying decrease in boil-off rate of the liquefied butane 12 the refrigerant supply rate to the condenser 6 can be progressively decreased. y r

If all of the liquefied butanelZ dissolves before the cargo has reached its destination, the supply of refrigerant to the condenser 6 may be maintained at a very low rate or terminated. In .either event, the. cargo comprising crude petroleum 10' and liquefied. butane 12 may be discharged using conventional cargo pumps. If a layer of liquefied butane 12 is present on the surface of the crude petroleum 10, the free butane may be removed by means of a gas lift system having a floating suction. An example of such a gas lift system is described and claimed in British Patent No. 869,080.

Referring now to FIGURE 2,.a center tank 1 is as before flanked by wing tanks 2 and .3, closed by means of a roof 4 and has a vent 5 giving access to a condenser 6. A retainer 11, in the form of a rectangular skirt open at the top and bottom, is positioned within the center tank 1 by means of stays 14 and 15 attached to the walls of the tank. Carrier liquid 10 is supplied as before to the center tank 1 and the wing tanks 2 and 3. Liquefied gas is supplied only. to the surface of the carrier liquid 10 within the retainer '11 to form a layer 12, floating on the surface of the carrier liquid but confined within the retainer 11. The retainer 11 keeps the liquefied gas 12 in compact form and also prevents chilling and embrittlemento'r' walls 13. The retainer. 11 may be made of any convenient material, such as metal or plastic, and may take the form of a fixed barrier or skirt, suspended from or attached to thereof or sides of the vessel containing the carrier liquid, or supported from the bottom of the vessel. The retainer 11 may be provided with perforations in its upper extremity abovethe level which the liquefied gas 12 is expected to reach and may also be provided with a perforated bottom plate which is useful for reducing premature mixing of the liquefied gas with the carrier liquid 10.

In FIGURE 3 there is shown a center tank 1 flanked by wing tanks 2 and 3, closed by means of a roof 4 and having a vent 5 giving access to a condenser 6. A retainer 17 in the form ofa floating pan having an imperforate bottom is restrained to move only in a vertical direction by runners or rollers 20'attached to the pan 17, said runners being directed by'guides 1S and 19 extending 10 is supplied as before to wing tanks 2 and 3 and to the center tank 1 and when the carrier liquid in center tank 1 has reached an appropriate level, liquefied petroleum gas 12 is supplied, for example by a swing-arm to the retainer 17. In this case, the liquefied gas 12 must be pumped out of the retainer 17 when the cargo comprising carrier liquid is discharged. It should be noted that with a pan instead of a skirt evaporation rates at equilibrium are somewhat higher. This is probably because of convection resulting from the increased density of the oil just below the pan; in this case there is no liquefied gas dissolving into the oil and hence no compensating reduction of density.

In FIGURE 4 there is shown means. for discharging liquefied petroleum gas from the surface of a carrier liquid. A layer 12 of liquefied petroleum gas floating on the surface of a carrier liquid 10 held in a cargo tank 1 is retained, as hereinbefore described for FIGURE 2, within a retainer 11. A floating suction 25 communicates by means of a conduit 26 with a gas lift system of the kind described in British Patent No. 869,080 comprising a cyclone chamber 27 having a vapor discharge port 28 communicating with a source of vacuum (not shown) and a liquid discharge port 29 communicating with a pump 30. The discharge side of pump 30 is connected to a conduit 31 whereby the liquid can be drawn off into a receiving vessel. Application of low vacuum (about one-fifth of an atmosphere) to the cyclone chamber 27 causes the liquefied petroleum gas 12 to be lifted into the cyclone chamber and then to the intake port of the pump 30. If desired, means for condensing gas formed during discharge of the liquefied gas 12 may be incorporated with the gas lift system.

I claim as my invention:

1. A method of storing liquefied petroleum gas as a layer floating on the surface of crude oil having a temperature substantially higher than the boiling point of the liquefied gas, a vapor pressure less than one atmos phere at the prevailing temperature and a density greater than that of said liquefied gas at its boiling point, said method comprising:

(a) providing a container having top, bottom and side walls and an open-ended retainer skirt situated within said container near the top thereof and extending over a minor fractional part of the container area;

(b) filling said container with crude oil to a level which closes off the lower end of said open-ended retainer skirt;

(c) filling said retainer skirt with liquefied petroleum gas and floating the latter on the crude oil;

(d) condensing any of said liquefied gas which vaporizes with a condenser positioned vertically above said retainer skirt; and,

(e) returning said condensate to the body 'of said liquefied gas.

2. A method storing liquefied gas comprising:

(a) providing a tank partially filled with a carrier liquid having a temperature less than C., a vapor pressure less than one atmosphere at the prevailing temperature, a density greater than that of said liquefied gas, and which is chemically inert toward said liquefied gas;

(b) supporting a body of said liquefied gas on said carrier liquid;

(c) confining said liquefied gas within an open-ended retainer which is situated inside said tank and has a smaller horizontal area than the tank; and,

(d) condensing any of said liquefied gas which vaporizes and returning the condensate to the body of said liquefied gas.

3. A method of storing said liquefied gas as set forth in claim 2 wherein said liquefied gas comprises liquefied petroleum gases and said carrier liquid is crude petroleum.

4. A method of storing liquefied gas as set forth in claim 2 wherein said liquefied gas is retained within a pan floating on the surface of said carried liquid, and said pan is constrained against lateral movement by guide means extending vertically upwards from the bottom of said tank.

5. A method of storing liquefied gas comprising:

(a) providing a tank partially filled with a continuous body of carrier liquid having a temperature less than 60 C., a vapor pressure less than one atmosphere at the prevailing temperature, a density greater than that of said liquefied gas, and which is chemically inert towards said liquefied gas;

(b) supporting a body of said liquefied gas directly on said carrier liquid and in communication therewith;

(c) confining the contact between said liquefied gas and said carrier liquid to a fractional part of the surface of said carrier liquid; and,

(d) condensing any of said liquefied gas which vaporizes and returning the condensate to the body of said liquefied gas.

References Cited by the Examiner UNITED STATES PATENTS MYER PERLIN, Primary Examiner.

ROBERT A. OLEARY, Examiner.

LLOYD L. KING, Assistant Examiner. 

1. A METHOD OF STORING LIQUEFIED PETROLEUM GAS AS A LAYER FLOATING ON THE SURFACE OF CRUDE OIL HAVING A TEMPERATURE SUBSTANTIALLY HIGHER THAN THE BOILING POINT OF THE LIQUEFIED GAS, A VAPOR PRESSURE LESS THAN ONE ATMOSPHERE AT THE PREVAILING TEMPERATURE AND A DENSITY GREATER THAN THAT OF SAID LIQUEFIED GAS AT ITS BOILING POINT, SAID METHOD COMPRISING: (A) PROVIDING A CONTAINER HAVING TOP, BOTTOM AND SIDE WALLS AND AN OPEN-ENDED RETAINER SKIRT SITUATED WITHIN SAID CONTAINER NEAR THE TOP THEREOF AND EXTENDING OVER A MINOR FRACTIONAL PART OF THE CONTAINER AREA; (B) FILLING SAID CONTAINER WITH CRUDE OIL TO A LEVEL WHICH CLOSES OFF THE LOWER END OF SAID OPEN-ENDED RETAINER SKIRT; (C) FILLING SAID RETAINER SKIRT WITH LIQUEFIED PETROLEUM GAS AND FLOATING THE LATTER ON THE CRUDE OIL; (D) CONDENSING ANY OF SAID LIQUEFIED GAS WHICH VAPORIZES WITH A CONDENSER POSITIONED VERTICALLY ABOVE SAID RETAINER SKIRT; AND, (E) RETURNING SAID CONDENSATE TO THE BODY OF SAID LIQUEFIED GAS. 